Better Fathers Have Smaller Testicles

Why do some men make lousy parents? Children need their fathers to be invested in their development yet some fathers choose to walk away. Why do these men make such a decision? A group of inquiring neuroscientists at Emory University in Atlanta wanted to know. In a recent paper published in the highly prestigious journal Proceedings of the National Academy of Sciences the qualities of human fathers were investigated with regard to the paternal nurturing behavior, and some important biological factors, including testosterone levels, brain activity and testicular volume.

In the past fifty years father absence has dramatically increased. An influential evolutionary theory, known as Life History Theory, suggests that there is a trade-off between the effort placed on parenting and mating. Essentially this theory states that since we only have a limited amount of energy to devote to mating or parenting we’re going to be really good at doing just one of them. I know that this sounds counter-intuitive but there are numerous examples that have been documented throughout the animal kingdom, including humans. The researchers at Emory University wanted to determine whether there were body and brain function characteristics that could explain why men make specific decisions regarding parenting.

Having lots of testosterone tends to induce males to mate but spend less effort parenting their offspring. In humans, having low testosterone levels usually leads to a reduced libido and less mating success. Married men who have high testosterone levels tend to divorce more often. Across many species, including humans, testosterone levels are inversely correlated with paternal involvement. These findings have led to the assumption that decreasing levels of testosterone suppress further mating efforts leading to more emphasis placed upon infant rearing.

Testicular size and function are also related to mating behavior. With regard to primates monogamous males have smaller testicles than do males living in large groups who participate in multiple partnering breeding arrangements. Testicular size in some species predicts reproductive success and overall rates of copulation. Evidence from the animal world has indicated that there is an inverse relationship between testicular size and parenting effort by males.

The recently published study involved 70 biological fathers between the ages of 21 and 55 who had one to four children. They reported that the fathers’ parental investment was inversely related to their testicular volume. Overall, smaller testicles and lower levels of testosterone were reliable predictors of whether the fathers would participate in more care-giving to their children. Obviously, we have a great deal more to understand about the relationship of these two factors.

© Gary L. Wenk, Ph.D., author of Your Brain on Food (Oxford, 2010)